I. Field of the Invention
The present invention relates to an X-ray image tube and, more particularly, to an X-ray image tube wherein a photocathode having different curvature radii at its central and peripheral surface regions is arranged at an input side of an evacuated envelope.
II. Description of the Prior Art
X-ray image tubes have been widely used to obtain X-ray images for medical diagnosis. A photocathode having a spherical or hyperbolic curved surface is used in a conventional X-ray image tube using an electron-optical focusing system.
A focusing surface of electrons emitted from this photocathode is not a flat surface but a curved surface with a considerably large curvature radius. An image on an output phosphor screen of the X-ray image tube is picked up by a television camera or an optical camera through an optical lens system. In addition to this reason, in order to simplify the fabrication of output phosphor screens, the output phosphor screen has a flat surface. When the focusing surface of the electrons from the photocathode is deviated from the flat surface, the focusing state of the output phosphor screen becomes poor. The resolution of the image formed on the output phosphor screen is degraded.
Assume that the photocathode has a spherical surface. When an X-ray image tube has its entire photocathode surface as an input field of view, the focusing surface becomes a relatively flat surface, and a good resolution can be obtained. However, an object is observed mainly from a central surface region of the input field of view. For example, when the entire input field of view of the photocathode has a diameter of 320 mm, a surface region having a diameter of 160 mm or 230 mm is frequently enlarged to a size corresponding to that of the entire input field of view. In this case, a trajectory of electrons emitted from the peripheral surface region of the input field of view having the diameter of 160 mm or 230 mm passes outside that of the electrons emitted from the peripheral surface region of the entire input field of view at the anode side of the electron lens system, thereby increasing the focusing action on the electron beams. As a result, when that surface region of the photocathode which has the diameter of 160 mm or 230 mm is enlarged and the image is picked up, the focusing surface of this region deviates from the flat output phosphor screen, thus degrading the resolution. In this case, when a photocathode has a hyperbolic curved surface whose opening extends toward the output side, the trajectory of the electrons in the peripheral surface region comes closer to be parallel to the axis of the electron lens. As a result, the trajectory of the electrons passes inside that of the electron beams emitted from the peripheral surface region of the entire input field of view. For this reason, the focusing action on the electrons is weakened, and the focusing surface of the region having the diameter of 160 mm or 230 mm becomes flat, thereby improving the resolution.
However, when the photocathode has a hyperbolic curved surface and an image of the entire input field of view is picked up, the trajectory of electrons emitted from the peripheral surface region of the entire input field of view comes close to be parallel to the axis of the electron lens. Therefore, the trajectory of the electrons passes through the peripheral surface region at the cathode side of the electron lens system. The beams are strongly focused, and the focusing surface of the entire input field of view greatly deviates from the output phosphor screen, thus degrading the resolution. Thus, when the photocathode of the type described above is used, the entire surface of the photocathode cannot be used as the effective field of view.
As described above, when the photocathode comprises a spherical surface and the entire surface thereof is used as an input field of view, a good resolution can be obtained. In order to observe an image in more detail, when only the central surface region of the photocathode is enlarged to focus an image on the phosphor screen, the image greatly deviates from the focusing surface, degrading its resolution. In order to reduce the error of the focusing surface when only the central surface region is enlarged and focused, a meridional curvature radius of a cross section of the photocathode is increased from the central surface region to the peripheral surface region like in a hyperbolic curved surface but unlike a predetermined spherical surface. When a surface is used such that an increment of the meridional curvature radius is larger than a constant derived from a linearity between the increment and a distance from the central surface region, a good resolution can be obtained in the case wherein the central surface region is enlarged and focused. However, when the entire surface of the photocathode is used as the input field of view, the resolution in the peripheral surface region is greatly degraded.